Instrumentation amplifiers are essentially devices that provide voltage gain for small signals that are frequently accompanied by common-mode noise. Instrumentation amplifiers are most often differential input, single ended output amplifiers. To be good voltage amplifiers, instrumentation amplifiers must place minimal loading on the signal source. For this reason, the gain-setting feedback elements are normally internal to the instrumentation amplifiers. Further, the gain -setting feedback elements must have good common-mode rejection, which places importance on tight device matching such that the gain-setting feedback elements are best implemented in integrated form.
Most such devices use operational amplifier buffers at each input. This produces very high input impedance as required, but demands high speed from the operational amplifiers so that the internal virtual nodes can remain pinned down over the frequency range of interest. The devices often use bipolar or BiCMOS processes to meet performance objectives, while some require laser trimming of on -chip resistors to meet accuracy requirements. This adds to the complexity of the integrated circuits
External resistors may be used to set gain. However, such resistors do not match internal integrated circuit resistors and have different temperature coefficients, which degrade accuracy.